Reset CSS settings from Javascript -

is there way reset settings of css element? have input want validate , not possible because set border of text input red can't come settings of css automatically. there way it? in advance.

edit 1

<input id="tm" type="text" onchange="validatestarttime()" name="orario_start" placeholder="orario inizio"> 

and validatestarttime() is

 function validatestarttime(){          var t_st=document.getelementsbyname("orario_start");         t_st[0].style.bordercolor="gray";//here want set css settings automatically         if(t_st[0].value.length==0){             t_st[0].style.bordercolor="red";             flag=false;         }         //validation time         var t1=t_st[0].value.split(" ");         if(t1.length!=2){             t_st[0].style.bordercolor="red";             flag=false;         }else{          var t1_1=t1[0].split(":");             if(t1_1.length!=2){                 t_st[0].style.bordercolor="red";                 flag=false;             }         }      } 

hi if using jquery can change css directly :

$('#yourelementid').css({     border : 1px solid #ff0000 }); 

if facing issue of removing red border again onmousedown can remove border.

 $('#yourelementid').mousedown(function(){      $(this).css({        border : 1px solid white      }); });